By Murat Ali and Andrew Wright
31st May 2016

It probably hasn’t escaped you that 3D-printing is ever more popular, with news agencies reporting on the latest medical, home office and aerospace innovations that utilise this fantastic technology. What might not be immediately obvious is the similarity between what you can do in your front room and what Boeing, Airbus, the European Space Agency, NASA and multiple others are doing using similar principles.

Think of words such as precise, expensive, high-level, complex and advanced. Cutting-edge, although an overused cliché, applies in all except the actual process. Printing in 3D is actually a very simple concept; just layers of material one after the other, gradually building up anything from simple to ridiculously complicated shapes. The process can be used to manufacture parts such as a critical jet turbine blade, or a simple part for your model aeroplane at home, made by you.

The basics of 3D-printing

The technology to do all of this now exists and all we need, aside from a moderate pile of notes with which to purchase the kit, is to know how to use it. So where better to start than in school. Think back to technology lessons involving traditional methods of woodwork, and (hopefully we’re not too old to say) more recently computer-based electronics and programming, which along with materials science are the basic building blocks of this new technology. Mix into this the current generation of engineers with Computer Aided Design (CAD) and manufacture (CAM) skills, and you have the basis of efficient design to production processes when it comes to 3D-printing.

So what exactly is this supposedly magical technology, and how does it work? Undoubtedly most of you reading this either own or have used a printer at some stage. Your printer effectively reads data from a PC, laptop or device that you sent the data from and physically prints that data: up, down, left and right, and across the page onto paper using ink. This can be referred to as two-dimensional (2D) printing. Now imagine you replace the ink with a material such as plastic or metal and the printer is now able to print that material: up, down, left, right, and away from the page. This exciting prospect is the basis of three-dimensional (3D) printing. Now we open up a huge opportunity to produce parts that can usefully be used on a model plane, and even parts of a jet engine. It works by starting from the bottom and building up layers of material as it moves it’s way up, like building a house from bottom to top, one brick at a time. It therefore seems logical to call this, ‘additive manufacturing’, because the material is added one step at a time.

This technology makes it possible to manufacture parts that conventional processes may not be able to achieve. It can make producing prototypes easier, especially if your prototype can be produce to a small scale. You could 3D print this on your desk and examine your idea as a 3D object rather than a flat image on your computer screen. You even have control over the porosity of the component, which is a technical word for describing how much empty space there is inside the part. This means you could make the part lighter and with less material than the same part machined from a solid piece of material.

Some challenges to overcome

There are of course some temporary drawbacks. We say temporary because, as the technology improves, we would expect these drawbacks to diminish. Some of the key challenges at the moment are:
– the availability of materials,
– producing parts as quickly and accurately as some current manufacturing methods,
– size.

At the moment it would be feasible to 3D print a complete model plane; however, the larger the part is the bigger the 3D-printer needs to be to print it.

Bearing this in mind, it is understandable that the current use of this technology for production of small batches, or even on mass scale, is in the making of relatively small parts. Being small may help to meet the safety regulations. As far as aviation is concerned, we don’t think of a Jumbo Jet as being particularly small, so you’d be right in thinking that 3D-printing a complete jetliner would not happen any time soon. In fact, imagining a 747 gradually appearing out of a giant printer the size of a multi-storey car park may be beyond even the most exaggerated science fiction minds. However, a Jumbo Jet, section of the International Space Station, Air Force fighter jet, or a weather balloon is made up of small parts, so this is good news for the status of the 3D-printing technology today.

Why is this good news? Do we just want to use 3D-printing for the sake of it? Well not necessarily, there are actually some spectacularly good advantages to it which we will come to later.

3D-printing for the skies

There is a fundamental engineering principle referred to as: KISS – Keep It Simple, Stupid. While a coarse phrase, it simply makes the point that the more complicated something is, the more chance something may go wrong. A component made of four parts, made in four different locations by four different machines, inevitably introduces more unknowns as a system made up of one part. Assuming each part is equally reliable, a Jumbo Jet with four engines has twice as much chance of having an engine failure as a twin-engined aircraft. So if possible, it would be ideal to have two enormous engines (or even one) but that introduces more issues, such as bending stresses, however we can talk about this another day. Reliability is a massive issue and consideration in Aerospace, as you can imagine. We can cover that another day too when we look at how to minimise unknown issues that may occur during manufacture. A quality program called Six Sigma can be used to minimise the defects in manufacturing and decrease the risk of something being made outside of the specification.

The point is that 3D-printing opens up the opportunity to produce parts, and improve safety and reliability in a way which may not have been previously possible, therefore complex multi-part systems may be replaced with parts which are less likely to fail. So in Aerospace we have new possibilities of simplify a system using 3D-printing; by taking a part that previously had 4 different bits due to the limitations of the manufacturing process, we may be able to ‘print’ it as a single part. We can do it in reverse. Instead of starting with a big bit of material and reducing it down with drills and so on and so forth, we start from nothing and build it up, layer by layer. Every layer is being laid from the ground up, effectively emerging up out of nothing, therefore we can avoid issues getting in to any sharp corners or producing complex shapes using big tools. Instead of subtracting, we are adding. Additive manufacture.

For 3D-printed parts to be used safely on an aircraft, what conditions do they need to withstand? The answer to this question largely depends upon what section of the aircraft the part is for, how long it is required, and what is the risk and consequence of failure. For a model plane you can afford to take more risk as the ultimate price to pay will be a crash landing in a field and maybe a plane bumping someone on the head if you are really unfortunate.

Let’s start inside the aircraft. For items used within the cabin (such as a table tray) the parts need to withstand the loads placed upon them by the passengers, cabin crew and pilots. last thing anyone wants is for a lap full of cottage pie and orange juice. The parts also need to withstand vibrations whilst the aircraft is operating and the impact loads during landing.

Now for the outside of the aircraft. Have you ever noticed icicles on your window, even when the sun is shining in the skies? The temperature outside the aircraft can be all the way down to around -75°C, depending on the type of aircraft and weather conditions at cruise altitude. Considering that water freezes at 0°C helps you appreciate how cold it really gets up there outside the aircraft. The power of flight means that we can travel across the world in reasonable amounts of time, but the plane rushes through the air at incredible speeds to achieve this, hitting the air particles along it’s way. If parts outside the aircraft are not designed with cold temperatures and aerodynamics in mind or are not able to withstand the aerodynamic forces, they may fail.

Moving from freezing temperatures to scorching hot temperatures, the jet engine. There is some good news here, 3D-printed parts are already being certified by the Federal Aviation Authority (FAA) in the US for use on Boeing aircraft. 3D-printing has allowed for complex shapes to be achieved which were unable to be achieved using traditional machining techniques. The fact that material is being built up means that there is very little waste of material which is good news for the environment and for those who pay the company bills. This really sets a precedent for future development and we can only hope it will continue in this way. For the air traveller; lighter, simpler and more reliable parts will hopefully lead to safer, more reliable and cheaper air travel in the future.

Safety in aviation is the number one priority. There simply is no market without confidence in safety. We only need to look at what happened to the aviation industry after September the 11th, or to the airline Pan Am; an industry that was massively successful but ultimately subject to events that undermined consumer, passenger and confidence. More than anything, we as members of the public empathise with those involved in such tragic events because flying for business or pleasure is such a commonplace and necessary factor in almost all of our lives.

Of course 3D-printing is not only for commercial aviation, because of the vast variety of aerospace applications we could talk about now. It is a huge market, and as we said above, anything likely to affect vast numbers of the travelling public is going to be subject to some pretty heavy safety regulation. The manufacture of aircraft is already subject to some of the closest scrutiny of any engineering production in the world, and the introduction of new parts and innovative processes will understandably be looked upon with extra suspicion and caution. Pilots in particular are highly risk-averse: that means we are suspicious of change because it introduces risk. Engineers are no different, but we must balance the need for improvement and innovation with reliable and well tested methods to ensure we are progressing in a safe way.

The future

Despite these challenges, the use of 3D-printing for producing parts for planes that take you to your wonderful holiday destination is still in its infancy, however, it is showing great promise. The idea that you can own and use a 3D-printer to produce parts yourself is also exciting. This could be one of the major innovations in the future of home computing.

You could use your 3D-printer to produce the parts that contribute to the next iteration of aircraft design, even if only for your model plane. This 3D-printer could inspire your son and daughter to make and fly the best-looking toy. Even if you decide not to do any 3D-printing yourself, it is likely that you will be enjoying your flying experiences surrounded by 3D-printed parts much sooner than you think.



You can read more and follow Murat Ali at where this article has also been posted. 


Today I read an article from the Wall Street Journal and it really struck a chord; something powerful which could really be used to our advantage in modern aviation. During my time spent working in different countries, I have been astounded by the varied manner in which normal professional business is conducted: even in multinational corporations where a ‘norm’ is more defined.

We have an industry of immense capability when it comes to data gathering, analysis and change management. We monitor everything. Technical professionals on the ground know when a modern airliner is a ‘bit sick’ long before the pilots do. Aviation is one of those areas in life where any possible prospect for advance is savagely pursued – not just because of the competitive nature of our industry, but because safety is in ALL our interests. Old or young, almost all of us owe our continued existence on this planet to a safely-executed commercial flight at some stage in our past; the safety of that particular flight was the layer of Swiss Cheese (see James Reason) successfully acting as a barrier between our risky willingness to hurl ourselves in to the air at high speed and the promise of a rather untimely demise as a result.

But how was the operation of that flight affected by the nationality of the crew and/or airline? How did the culture associated with that operation affect our successful continuance? In his book Outliers, Malcolm Gladwell speaks about the turnaround in Korean Air and how culture was a key factor in the process for change. He also speaks about the mathematical abilities of the Chinese, and how that could be attributed to the way they are required by the rules of their language to describe items in a certain way.

This brings me back to the Wall Street Journal article at

“In one study, Spanish and Japanese speakers couldn’t remember the agents of accidental events as adeptly as English speakers could. Why? In Spanish and Japanese, the agent of causality is dropped: “The vase broke itself,” rather than “John broke the vase.” “

So what’s my point, I hear you ask?

Aviation relies on a Just Culture: no blame, simply prevention. How is this demonstrated in cultures where the language refers to events in a blameless manner, as reported in the link above, versus cultures where the language is more blame-oriented or individual-specific?

What about this:

“Simply showing that speakers of different languages think differently doesn’t tell us whether it’s language that shapes thought or the other way around. To demonstrate the causal role of language, what’s needed are studies that directly manipulate language and look for effects in cognition.


“One of the key advances in recent years has been the demonstration of precisely this causal link. It turns out that if you change how people talk, that changes how they think. If people learn another language, they inadvertently also learn a new way of looking at the world. When bilingual people switch from one language to another, they start thinking differently, too. And if you take away people’s ability to use language in what should be a simple nonlinguistic task, their performance can change dramatically, sometimes making them look no smarter than rats or infants. (For example, in recent studies, MIT students were shown dots on a screen and asked to say how many there were. If they were allowed to count normally, they did great. If they simultaneously did a nonlinguistic task—like banging out rhythms—they still did great. But if they did a verbal task when shown the dots—like repeating the words spoken in a news report—their counting fell apart. In other words, they needed their language skills to count.)”

Multi-tasking involving linguistic processing is difficult. During my basic pilot training I was taught the concept of “divided concentration” – isolating individual tasks, prioritising and then rapidly switching between them in order to achieve them all… just not at the exact same time. Example: monitoring airspeed, rate of change of altitude, nose attitude and heading, all at “the same time” when in reality it was achieved through a series of small checks one after the other. *When looking for a house number do you turn the radio down? If so, is because there is speech involved? Would the distraction be as great if solely instrumental – e.g. classical – music was playing? How does that apply to the use of hands-free equipment in a car and what about those people who suggest that listening to music is the same as having a conversation?


“Language is a uniquely human gift. When we study language, we are uncovering in part what makes us human, getting a peek at the very nature of human nature. As we uncover how languages and their speakers differ from one another, we discover that human natures too can differ dramatically, depending on the languages we speak.”

…is it therefore ever more important that aviation uses a common language (currently English) and extrapolates that use all the way to the safety management system rather than just the actual flying operation? Does our mother-tongue and main language culture determine how prone we are to certain behaviours, be it in the flight deck, in the airline management or even the regulatory body?

Of course the water is muddy on how to approach this. Changing the culture within Korean Air was a massive task in itself. But spreading cultural change across many nations many be impossible to achieve, as shown with the difficulties encountered in the implementation of the Single European Sky project.

But one thing is clear: a better understanding of how our minds work culturally will lead to better training, better development and implementation of Safety Management Systems, and a wider acceptance that there is real power to be gained through identifying the different traits of different cultures, without being labelled discriminatory or racist in the process. By jumping to conclusions and unnecessarily stereotyping we can really harm our society; but by understanding the innate differences in our culture-born abilities, we can adapt our management systems to make for a safer, stronger commercial aviation culture that acknowledges the one common need across all backgrounds: safety in air travel.

It is not my intention to purposely identify Air France as a safety culprit when it comes to dodgy approaches to land. But they have certainly been providing the industry with plenty to talk about in recent times. And this is a good thing; not because we like pointing fingers – quite the opposite – but because there is so much to learn through casual discussion of incidents and accidents.

(The benefits of casual discussion are also clear when preparing for six-monthly simulator training sessions, in which all manner of unfortunate and sometimes unpleasant events are thrown as us. Pilots yet to meet their simulated “bad day” question their colleagues, beady-eyed and attentive to the smallest nugget of advance warning, on the ‘events’ handled by those who have already suffered in the recent past. This talking encourages study and formation of opinion and is a critical aspect to the continuous development we must commit to as professional aviators.)

So why have I picked up on Air France?

I will leave the actual judgement and discussion up to you, but I want to identify a couple of common factors in some fairly high-profile approach instability events as reported via

See if you can spot the common factors in each:

  1. High energy approach (too fast or high to safely get on – and stop on – the runway)
  2. Issues with autopilot and a tendency to disconnect it rather than operate it appropriately
  3. Mishandling of the resulting situation
      1. either physical mis-manipulation of the controls, or
      2. improper monitoring of the aircraft systems; no observation by the crew of what the aircraft was screaming at them.

Report: Air France A320 at Tel Aviv on Apr 3rd 2012, approach to stall on turning final results in Alpha Floor and flaps overspeed

Report: Air France A388 at New York on Oct 11th 2010, oversped flaps during go-around

Report: Air France A319 at Tunis on Mar 24th 2012, extreme rate of descent on glideslope intercept, GPWS alerts and descent below safe altitude

Report: Air France B772 at Paris on Nov 16th 2011, continued to descend despite go-around

(As an addition, I have already written about another Air France event on approach to Paris here – false glideslope capture and near stall.)

None of these resulted in injuries. I have paraphrased from the avherald reports.

Tel Aviv

“The pilot flying however felt they were high on the approach but did not share his concern with the pilot monitoring.

“Still in the decent through about 1540 feet the autopilot gets disconnected, flight director and autothrust remain engaged.

“The pilot flying applies nose up inputs for about 10 seconds while the flight director commands nose down inputs to maintain the target speed, the airspeed reduces from 135 KIAS to 122 KIAS with the pitch increasing from 0.7 to 10 degrees nose up, the pilot monitoring later provided testimony that he was monitoring the alignment with the runway. An automatic “Speed, Speed, Speed” call activated at Vapp-16 knots.

“The pilot flying decided to go around but did not call out the go-around. The pilot flying moved the throttle levers into the TOGA detent and applied nose up inputs, the pilot monitoring applied nose down inputs for about 2 seconds (dual input). At that point Alpha Floor protection activated applying TOGA thrust and TOGA Lock, 3000 feet is being selected into the altitude window, open climb mode is being engaged, the speed returns into normal range, the pilots do not detect the “TOGA LOCK” status however. The aircraft climbs through 2000 feet, the crew recognizes difficulties in reducing the thrust. The flaps are selected to 1, the speed increases to 208 KIAS and still continues to increase, 2000 feet and 188 KIAS are being selected into the MCP, the aircraft climbs to 2500 feet before starting to descend again, the speed increased to 223 KIAS (flaps limit 215 KIAS), an overspeed alarm activated.”

New York

“The first officer was pilot flying using the autopilot but did not engage the approach mode. As result the aircraft was above glidepath, the first officer disengaged the autopilot and continued manually.

“At 480 feet AGL the speed was still 210 KIAS and above glidepath about 1nm before the threshold, when the captain, pilot monitoring, ordered a go-around surprising the first officer. The thrust levers were moved to the TOGA detent and initiated a go around, the aircraft quicky assumed a climb rate of about 3400 fpm, pitch attitude about 2 degrees nose up. The first officer moved the thrust levers to the MCT detent, unnoticed by the captain. The aircraft accelerated exceeding the maximum speed for the flaps at position 2, an alert sounded and the flap relief moved the flaps to position 1, about 2 seconds later the flaps were selected to position 1, the vertical speed increased to +4200 fpm. The aircraft climbed through the go-around altitude of 1000 feet, at 1600 feet the first officer attempted a first level off, the speed rose through 301 KIAS. Now the thrust levers were pulled into the CLB detent, which effectively commanded the autothrust into speed mode, the engines were spooled down to idle.”


“Upon contacting Tunis approach the controller advised that the active runway had just been switched to runway 19, which shortened the flying distance to land by about 20 nautical miles. The captain briefed for an ILS approach to runway 19 and decided to continue the approach.

“The aircraft descended through FL100 13.5nm before touchdown, the autopilot was disconnected.

“Descending through 3550 feet, 1700 feet above glide, about 5nm from touchdown, flaps still at position 0, vertical speed -4400 fpm, speed brakes and landing gear extended, the first officer (ATPL, 1,700 hours on type) transmitted they were established on the ILS 19. The autothrust was disconnected, the engines were reduced to idle thrust.

“The captain re-engaged autothrust and autopilot.

“The captain stowed the speed brakes and disengaged the autopilot again.

“The thrust levers are placed into the TO/GA detent, the aircraft turns to the left and climbs to 2000 feet, then positions for a visual approach to runway 19 with ILS support.”

Paris (Number 1)

” “Go-around”, the captain responded by pushing the throttles forward to initiate the go-around disconnecting autothrottle in the process. A nose up pitch command on the control yoke is recorded however insufficient in strength to disconnect the autopilot. While the aircraft began to accelerate the attitude changed from +1.15 degrees to -0.5 degrees. The captain ordered the flaps to be reduced to 20 degrees, the pitch decreases further to 2 degrees nose down. The relief pilot called out “Pitch!” 10 seconds after the go-around was initiated both crew pulled the yoke now resulting in the autopilot disconnecting, the aircraft pitched up sharply resulting in +1.84G vertical acceleration, the attitude changed from 2 degrees nose down to 7 degrees nose up and subsequently reducing to 4 degrees nose up when the control yoke was returned to neutral, speed was now 169 KCAS. The relief pilot again called “Pitch!”. The crew applied nose up input on the control yoke, the aircraft reached its lowest point of 63 feet AGL at 180 KCAS, the nose rose to 11 degrees nose up in 2 seconds and subsequently 19 degrees nose up and the aircraft climbed out to safety.”

Paris (Number 2)

“When the aircraft was 9nm before the runway threshold, the aircraft had been established on the localizer however was at 4950 feet MSL about 1750 feet above glideslope. At 4nm before touchdown the aircraft was at 3700 feet MSL: 2100 feet above glideslope, the glideslope indications in the cockpit had already reverted to the side band of the glideslope transmitter (mirror glideslope at 9 degrees). At about 2nm out, the aircraft descended through 2850 feet MSL 1600 feet above glide, the vertical channel of the autopilot mode changed to glideslope capture.

“When the aircraft rotated through 26 degrees nose up, the crew disconnected the autopilot and pushed the side stick forward to near the mechanical stop, the pitch attitude and the rate of climb reduces, the aircraft began to accelerate to 143 KIAS again, the autothrust system disconnected. About 30 seconds later the crew re-engages autopilot 1 and autothrust in climb thrust with the intention to perform an automatic go-around.”

* Note: in the second Paris event, the crew recognised the incorrect flight mode of the autopilot and took control manually once again. This showed good decision-making, ironically saving a situation caused by very poor decision-making in the first place. However, it can be argued that it was exceptionally clear to the crew that the wrong flight modes were engaged, because the aircraft had suddenly nosed over and was now pointing at the earth.

Happy reading and opining.

This decision-making tool was taught to me as a framework during my first jet training. DODAR refers to Diagnose, Options (& Consequences), Decide, Assign Tasks, Review. Working this process in a cycle can help to make a plan when things are not going quite as expected. Important: Review, often overlooked. This ensures that the plan changes if the situation changes.


OK, it is fair to say that the low cost carriers are seeing their methods replicated, particularly in short-haul, so that could be your first thought. But actually I refer to safety trends. In the never-ending battle of man versus machine, occasionally versus ground (a battle which we ironically sometimes try to win using yet another machine), who is winning?

Draw your own conclusions with statistics recently published at

In global terms, the accident rate has been declining steadily ever since the 1950s. In 2000, the concern was that, even with that encouraging trend, the growth of the airline industry would result in an absolute increase in the number of fatal accidents occurring each year. Some categories of accident were proving to be a particular challenge. In the late 1990s, the Flight Safety Foundation’s campaign to reduce controlled flight into terrain (CFIT) accidents, which included their Approach and Landing Accident Reduction (ALAR) tool kit, appeared initially to have achieved some success. However, in 1999-2001 the CFIT rate started to rise again, causing concern in the flight safety world. Then, the widespread introduction of TAWS began to make a real impact on the number of CFIT accidents, and the decline in the accident rate resumed.

Improvements in overall safety, and the reduction in the accident rate seen in the period 2002-2007 can be attributed to safety enhancements made possible by digital technology such as FDMACASTAWS, etc but in recent years it appears that the accident rate has again levelled out.

For large commercial airliners, a small number of accidents account for the majority of fatalities each year.

This refers to (and explains in more detail) enhancements such as:

  • FDM: Flight Data Monitoring. Flight parameters are downlinked from the aircraft and analysed against limits – throwing up ‘red flags’ where aircraft or operator limitations are exceeded. Can be used to analyse trends and investigate safety reports, as well as identify deliberate violations.
  • ACAS: Airborne Collision Avoidance System. Often referred to as TCAS (Traffic Collision Avoidance System). If something in the system, be it air traffic control, the autopilot, the weather or the pilots, screw up, this will identify airborne threats to the aircraft – i.e. warn of an impending collision which you might have missed.
  • TAWS: Terrain Awareness and Warning System. Often called GPWS or Ground Proximity Warning System. Warns you if you are, in the estimation of the system, likely to hit the ground in an abnormal manner.

So what is the point in mentioning the above safety enhancements, which have been around for some time now?

Let’s think about what they are protecting us against. It can be said that FDM acts significantly in identifying trends towards wilful violation of limitations by operating crew, such as continuing an unstable approach unstable below 500ft, and is a disincentive. Trends identified with FDM can be addressed through training i.e. by improving awareness. ACAS and TAWS also act to improve situational awareness which may be degraded through distractions, abnormal situations, fatigue or lack of proficiency.

These tools are acting as a sixth-sense to identify threats we might have missed. In the case of FDM, if being used for investigation of safety reports then the threat may be hidden in the trend (e.g. a trend towards continuing unstable approached below 500ft may suggest a lack of awareness of the fact that this is the single highest contributory factor in runway excursion events). If being used to stop violation of operational limits then the threat is in the lack of awareness of why those limits exist. These tools are eyes and ears which acts to keep us safe when ours, be it our actual eyes and ears or be it our own sixth sense built up from experience of real life and/or training, do not.

Now if the accident rate has levelled out, what is the problem with these tools? They have helped to reduce it, but at some stage we will no doubt start to rely on these tools too much. It has been said that wearing seatbelts, having airbags and improving the design of cars has not, long-term, improved personal safety since it has invoked a sense of invincibility amongst drivers and lead to more risk-taking than previously.

Perhaps we just get used to the system, however safe it may be? That would mean our complacency is directly proportional to how safe we feel. That being the case, one has to ask the question… how safe do you feel? And why?

“Look at it like this: you are working as a crew to keep the aircraft and it’s contents safe. You are not protecting your buddy by not saying anything in the hope that you are protecting him… we are present, we are sitting there waiting for the PNF to highlight it and for it to be corrected. So effective monitoring means that the PNF has got to be on top of the game all the time. And because, basically you have more capacity than the pilot flying, you should always be one step ahead of him. Thinking he should be doing this now, why isn’t he doing it. Speed’s got a bit slow – oh that’s alright, he’s just moved the thrust up. Speed’s now 2kt slow, he still hasn’t moved the thrust up – “SPEED”. You’re not bringing it to anyone else’s attention, be it instructors, passengers, Flight Data Monitoring systems.. that he’s flying two knots slow. You’re bringing it to their attention that you’re working as a crew.”

Asiana Airlines visual approach, San Francisco. Image courtesy of

Asiana Airlines visual approach, San Francisco. Image courtesy of